The measurement of dielectric constant (or dielectric permittivity) of formations surrounding a borehole is known to provide useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, 2.2 for oil, 7.5 limestone, and 80 for water), so measurement of dielectric properties is a useful means of formation evaluation.
A logging device which measures formation dielectric constant is disclosed in the U.S. Pat. No. 3,944,910. The logging device includes a transmitter and spaced receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations is determined from the receiver output signals. The dielectric constant and, if desired, the conductivity of the formations can then be obtained from the phase and attenuation measurements. Measurements are typically, although not necessarily, made on the formation invaded zone. Two transmitters are generally used in a borehole compensated array to minimize the effect of borehole rugosity, tool tilt, and dissimilarities in the transmitters, receivers, and their circuits. [See, for example, U.S. Pat. No. 3,849,721.]
The antennas shown in U.S. Pat. No. 3,944,910 are slot antennas, each having a probe that extends across the slot in a direction parallel to the longitudinal direction of the borehole. This configuration has become known as a "broadside" array. The U.S. Pat. No. 4,704,581 describes a logging device of similar type, but wherein the slot antennas have probes that extend in a direction perpendicular to the longitudinal direction of the borehole. This configuration has become known as an "endfire" array. The endfire array exhibits a deeper depth of investigation and is less affected by tool standoff (e.g. from mudcake) than the broadside array. On the other hand, the broadside array exhibits a stronger signal characteristic than the endfire array and may be preferred in relatively lossy (low resistivity) logging environments.
A logging device which utilizes teachings of the above-referenced U.S. Pat. Nos. 3,944,910 and 4,704,581 is the electromagnetic propagation tool ("EPT"--mark of Schlumberger). A so-called adaptable EPT ("ADEPT") can provide either broadside operation or endfire operation during a given logging run, depending on conditions. The ADEPT logging tool has two changeable pads, one containing a broadside antenna array and the other an endfire antenna array.
Notwithstanding the described advances in electromagnetic pad devices which determine permittivity and conductivity of the formation invaded zone, in various circumstances difficulties can arise in the obtainment of accurate measurements. Consider, for example, the simplified models of FIGS. 2A and 2B. In FIG. 2A an EPT type of pad 250 is shown adjacent a standoff layer 215 (e.g. a mudcake) and formations 220. The pad 250 has transmitters 251, 252 and receivers 256, 257 in a conventional EPT borehole compensated arrangement. The formations have a dielectric permittivity .epsilon..sub.f ' and a conductivity .sigma..sub.f, and the standoff layer has a thickness h.sub.m, a dielectric permittivity .epsilon..sub.m ', and a conductivity .sigma..sub.m. Typically, the measurements taken by the EPT device (of which pad 250 is a part) include the attenuation and phase of microwave electromagnetic energy propagating between the transmitters and receivers. These two measurements are insufficient to determine the five unknowns (h, .epsilon..sub.f, .sigma..sub.f ', .epsilon..sub.m ', and .sigma..sub.m) or even four unknowns in cases where h is known from other measurement(s). Accordingly, an approach that has been taken is to consider the standoff layer and formations as a composite medium, labelled 230 in FIG. 2B, having a dielectric permittivity .epsilon..sub.c and a conductivity .sigma..sub.c. This approach can result in incorrect conclusions about the nature of the formations, the standoff layer, or both.
It is among the objects of the present invention to provide an improvement in the determination of properties of the formations and the standoff layer in the propagation logging of earth boreholes. It is also among the objects of the present invention to provide improved techniques and equipment for the accurate determination of properties of subsurface formations using electromagnetic energy having a magnetic dipole characteristic.